Prefixes are useful for expressing units of physical quantities that are either very big or very small. If you write out such big or small numbers, it will be time consuming and prone to errors.

Some of the Greek prefixes and their symbols to indicate decimal sub-multiples and multiples of the SI units are:

Number | Prefix | Number | Prefix |
---|---|---|---|

$10^{-9}$ | nano (n) | $10^{-1}$ | deci (d) |

$10^{-6}$ | micro ($\mu$) | $10^{3}$ | Kilo (K) |

$10^{-3}$ | milli (m) | $10^{6}$ | Mega (M) |

$10^{-2}$ | centi (c) | $10^{9}$ | Giga (G) |

**Common examples:**

- 1 cm = 0.01 metre
- 1 kg = 1000 grams
- 1 GB = 1 000 000 000 bytes
- 0.000 0052 s = $5.2 \, \mu \text{s}$ = $5.2 \times 10^{-6} \, \text{s}$

$5.2 \times 10^{-6} \, \text{s}$ is what is known as **standard form**. When numbers are too large or too small, it is convenient to express them in standard form in the following format:

$$\text{M} \times 10^{\text{N}}$$

, where:

$\text{M}$ is in the range of: $1 \leq \text{M} < 10$

$\text{N}$ denotes the **order of magnitude** and is an integer.

Order of magnitude are used to estimate numbers which are extremely large to the nearest power of ten. The table below shows how orders of magnitude are used to compare base quantities – mass and length.

Mass ($\text{kg}$) | Factor | Length ($\text{m}$) | Factor |
---|---|---|---|

Electron | $10^{-30}$ | Radius of proton | $10^{-15}$ |

Proton | $10^{-27}$ | Radius of atom | $10^{-10}$ |

Ant | $10^{-3}$ | Height of ant | $10^{-3}$ |

Human | $10^{1}$ | Height of human | $10^{0} = 1$ |

Earth | $10^{24}$ | Radius of Earth | $10^{7}$ |

Sun | $10^{30}$ | Radius of Sun | $10^{9}$ |

Order of magnitude is useful when you do “back-of-the-envelope” calculations. A back-of-the-envelope calculation is a rough calculation, typically jotted down on any available scrap of paper such as an envelope.

Back-of-the-envelope calculations are used to quickly check something. If your back-of-the-envelope calculations yield several orders of magnitude bigger or smaller than what you expect, your formula or input variables must be wrong.

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